The present invention relates generally to stability control systems for automotive vehicles, and more particularly relates to preventing a rollover propensity in an automotive vehicle.
Stability control systems, either for yaw stability or roll stability, detect an instability or potential instability in a vehicle""s dynamics and modify the dynamics to ensure stability. Typically, such stability control systems utilize a controller receiving various inputs from sensors such as a yaw rate sensor, a speed sensor (wheel and/or vehicle speed), a lateral acceleration sensor, a roll rate sensor, a steering angle sensor, a longitudinal acceleration sensor, and a pitch rate sensor. The controller may also utilize other inputs, which can be sensed, inferred or estimated.
The stability control system generally acts when the vehicle is performing a dynamic maneuver such as a high speed turn or slalom maneuver. Typically, the controller of the stability control system will send a control input signal to another control system such a brake control system (such as ABS), a powertrain system, or a steering control system (when the vehicle is capable of controlled steering). For example, a particular wheel may be actively braked, its torque reduced, or its steering angle adjusted. One exemplary stability control system is described in U.S. Patent Application Publication US 2001/0008986 A1, the disclosure of which is hereby incorporated by reference in its entirety.
In this way, the stability control system will modify the tire force vector on the selected wheel or wheels to promote vehicle stability. When stability is reached, the stability control system removes the control input, leaving the tire force vector unmodified. Unfortunately, these systems can result in a force being impulsed back into the wheel after modification of the tire force vector. The increase in force on the wheel acts as a subtle trip (as if hitting a small curb), thereby inducing a rapid body roll movement characterized by a high roll rate or momentum. Clearly, this can lead to an increased propensity for rollover.
Accordingly, there exists a need to provide a stability control system and related method which decreases the propensity for rollover and prevents a rapid body roll movement after active management by the stability control system.
The present invention provides a method for controlling stability in a vehicle comprising the steps of recognizing a propensity for vehicle instability, modifying the tire force vector of the vehicle""s wheel to increase stability, and returning the tire force vector at a limited rate to prevent an impulse force on the wheel. Generally, the tire force vector is changed from a first unmodified state to a modified state, and then returned to a second unmodified state. The second unmodified state may be identical to the first unmodified state, or may alternately be different based on a change of conditions such as vehicle speed, wheel speed, or tire angle as determined by user inputs. The method may further comprise the step of holding the tire force vector in the modified state when the propensity for vehicle instability is no longer recognized.
The step of modifying the tire force vector is typically based on a proportional control signal, and may also be based on a derivative control signal and a double derivative control signal. For example, in roll stability systems the control signals represent roll angle, roll rate, and roll acceleration, respectively. Alternately, the proportional control signal is a yaw rate error of the vehicle, and the derivative control signal is the derivative of the yaw rate error of the vehicle (i.e. for a yaw stabilizing program). Preferably, the step of returning the tire force vector includes sending an interrupt signal that replaces the proportional control signal. Alternately, the step of returning the tire force vector will include sending an override signal that is summed with the proportional control signal.
The method may further comprise the step of determining a propensity for the impulse force on the wheel. This can be performed to determine whether the limited recovery rate of the tire force vector should be applied. Typically, this determining step of the impulse force is based on wheel speed and yaw rate of the vehicle. Further, other inputs can be utilized to identify when the limited recovery rate should be applied, including brake force, tire lateral force, tire steer angle, roll angle, and roll rate.